VHF satellite based radar antenna array

ABSTRACT

A space based radar antenna array is provided which includes a loop antenna, a narrow cylindrical central support column, and an erectable support structure for connecting the support column to the loop antenna. The loop antenna, the support column and the support structure together form a generally umbrella-like apparatus which is erected to deploy the antenna. When deployed, the loop antenna assumes a circular shape oriented in surrounding relation to the central support column. In the non-deployed state, the loop antenna is collapsed adjacent to the support column. The loop antenna comprises a plurality of printed circuit, end-fire YAGI-UDA antenna units.

FIELD OF THE INVENTION

The present invention relates to radar antenna arrays and in particularto a space based radar antenna array of generally umbrella-likeconstruction.

BACKGROUND OF THE INVENTION

There currently is interest in having low frequency directional antennasin space. Two design concepts for such an antenna have gained attention.One is a flat phased array. The other is a parabolic dish whichmechanically scans. The flat array has the disadvantage that when onesteers its beam to the horizon (as one must do to avoid directreflections from the earth) it sacrifices power (loses effectiveaperture), about 3 db worth. The parabolic antenna, in order to scan,must be constantly in motion, and this motion must be controlled to anextraordinarily fine degree in order to maintain the antenna properlyoriented, and hence effective. This is an extraordinarily difficultengineering task, the solution to which would be a complicated andexpensive control system.

Large diameter deployable loop antennas have been developed for use inspace. For example, U.S. Pat. No. 4,811,033 (Ahl et al) discloses aspace-based antenna generally in the shape of an umbrella wherein the"umbrella" surface comprises parabolic RF reflectors. The antenna isdeployed from a stowed configuration the surface contour of the loopantenna elements is automatically controlled. U.S. Pat. No. 4,578,920(Bush et al) is directed to a space-based antenna including acollapsible-expandable supporting truss structure. Other relevantpatents include U.S. Pat. No. 4,757,323 (Duret et al), U.S. Pat. Nos.4,658,261 (Reid et al) and 4,814,784 (Pallmeyer.)

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to provide a radar arraycapable of deployment in space, with a fixed orientation in space.

Another object is to permit such an antenna array to scan the horizonwithout the loss of power associated with flat arrays.

In accordance with the invention, a space based radar antenna array isprovided which includes a loop antenna, a cylindrical support columnhaving first and second ends, a connecting means, including a pluralityof support boom assemblies, for connecting the first end of the supportcolumn to the loop antenna, and means for moving the boom assembliesbetween a first position, wherein the boom assemblies extendsubstantially parallel to the support column such that the loop antennais disposed in a collapsed state in the vicinity of the support column,and a second position wherein the boom assemblies extend outwardlysubstantially perpendicular to the support column such that the loopantenna is fully deployed in a circular configuration.

In a preferred embodiment, the loop antenna further comprises aplurality of printed circuit, end-fire YAGI-UDA antenna units.Preferably, the connecting means further comprise a generally circularrunner slidably mounted to the support column such that the runner canmove along the support column from the first end of the support columnto the second end of the support column, the boom assemblies eachfurther comprise an extensor boom pivotally attached to the runner, asecondary boom connecting the top of the support column to the center ofthe extensor boom, and a main supporting boom connecting the center ofthe secondary boom to the loop antenna.

Because of its circular geometry, the array does not have the same powerfall off when scanning the horizon as does a flat array. Because thearray is physically fixed in space, and its beam steerable, it need nothave the complicated positioning hardware that would be needed for amechanical scanning system.

Other features and advantages of the invention will be set forth in, orbe apparent from, the detailed description of the preferred embodimentsof the invention which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side elevational view of an embodiment of the inventionin a deployed configuration.

FIG. 2 shows a side elevational view of the embodiment of FIG. 1 in aretracted configuration.

FIG. 3 is a view in the direction of lines 3--3 of FIG. 1, showing theembodiment of FIGS. 1-2 in a deployed configuration.

FIG. 4 shows a perspective view of one deployed antenna array segmentconstructed in accordance with the invention.

FIG. 5 shows a perspective view of one deployed antenna element cellconstructed in accordance with the invention.

FIG. 6 shows a schematic of the RF circuit of an embodiment of theinvention.

FIG. 7 shows a schematic of the transmit/receive module of an embodimentof the invention.

DETAILED DESCRIPTION

Referring to FIGS. 1 to 3, an embodiment of the Space Based Radarantenna array of the invention includes, as shown fully deployed in FIG.1, a narrow generally cylindrical support column 10, a loop antennastructure 12 (best seen in FIG. 3), a runner 20 which is adapted formovement along support column 10, and a plurality of radial supportingstructures, generally denoted 15, each comprising a main boom 14, asecondary boom 16, and an extensor boom 18. A propulsion system 22 isprovided at one end of support column 10 while a nuclear power system 24is provided at the opposite end of support column 10. FIG. 2 shows thearray in a retracted configuration.

Runner 20 is slidably mounted on the support column 10 such that runner20 can move along support column 10 from a first position near thepropulsion system 22 (as shown in FIG. 1) to a second position near thepower system 24 (as shown in FIG. 2) Extensor boom 18 is pivotallyattached to runner 20. Secondary boom 16 is, at one end, pivotallyattached to support column 10 near propulsion system 22 and, at theother end, pivotally connected the center of extensor boom 18. Mainsupporting boom 14 is, at one end, pivotally mounted to the center ofsecondary boom 16 and, at the other end, connected to the loop antenna12. Therefore, the support column 10, loop antenna 12, radial connectingstructure 15, and runner 20 generally describe or constitute anumbrella-like apparatus.

In the deployed configuration of FIG. 1, runner 20 is disposed near thepropulsion system 22, as illustrated. Extensor boom 18 projects outwardfrom, and generally perpendicular to, support column 10. Secondary boom16 and main boom 14 extend outwardly at an angle from support column 10.In the deployed state, loop antenna 12 describes a circular loopgenerally perpendicular to, and widely spaced from, support column 10.

In the retracted configuration shown in FIG. 2, runner 20 is disposednear the power system 24, extensor boom 18, secondary boom 16, and mainboom 14 are disposed parallel to support column 10, and loop antenna 12is collapsed in the vicinity of the power system 24.

In an exemplary, non-limiting example, support column 12 has a length of21 feet, and loop antenna 12 has a diameter of approximately 150 feet inthe deployed configuration and 13.4 feet in the retracted configuration.

Deployment of the antenna array 10 from the retracted configuration ofFIG. 1 to the deployed configuration of FIG. 2 is accomplished asfollows. Runner 20 moves from near the power system 24 toward thepropulsion system 22. Extensor boom 18 pivots with respect to the runner20. Likewise, secondary boom 16 pivots, at one end, with respect to thesupport column 10, and at the other end, with respect to the extensorboom 18. The combined movement of extensor boom 18 and secondary boom 16extends main boom 14 and thus deploys loop antenna 12. Runner 20 can bemoved along the support column 10 by an activator unit, indicatedschematically at 26 in FIG. 1, which can comprise a spring loadedactuator located in the main support column 10 and triggered by apyrotechnic device (not shown).

Referring to FIGS. 3-5, the structure of the antenna loop 12 will now bemore fully described. Loop antenna 12 comprises a plurality ofindividual antenna segments 30 with FIG. 4 showing one such antennasegment. In an exemplary embodiment, loop antenna 12 comprisestwenty-two antenna segments 30. Each antenna segment 30 furthercomprises a plurality of antenna cells 40 and FIG. 5 shows one such cell40. In the exemplary embodiment under consideration, each antennasegment comprises sixteen antenna cells configured as shown in FIG. 4with the antenna segment 30 comprising a two by eight array of antennacells 40. As shown in FIG. 4, four transmit/receive modules 32 aremounted along each main boom 14 and are each connected to upper andlower rows of antenna cells 40 (the two by eight array referred toabove). The rows are vertically spaced by support walls 34 disposedtherebetween and cables 50 connect the modules 32 to the antenna cells40 as is described in more detail below. (When speaking about thecomponent parts of the array, the terms "horizontal" and "vertical," donot refer to the earth and its horizon, but rather to segments 30, andthe cells 40 within segments 30. The "horizontal" plane for each segmentis that in which both members 14 lie, and the "vertical" plane one whichis orthogonal to this horizontal plane. As is seen in FIG. 4, individualcells 40 are oriented symmetrically with respect to these vertical andhorizontal directions. Cells 40 also generate two signals polarizedorthogonally to one another, and which are also symmetric to thesedirections.)

Each antenna cell 40 further comprises a plurality of printed circuit,end-fire YAGI-UDA antenna units 42. Preferably, four such antenna units42 are combined in the shape of a rectangular box to form one antennacell 40 (FIG. 5). Two opposing antenna units 42 provide verticalpolarization. The other two opposing antenna units 42 provide horizontalpolarization. The openings of cells 40 are square, and thisconfiguration produces equal vertical and horizontal beamwidths. Thebase material of each antenna unit 42 comprises bonded nylon fiberreinforced cells or polyester-glass fiber cloth. The sides of eachantenna unit 42 are affixed, for example, sewn together to produce arectangular collapsible box configuration.

Conducting elements are provided on the face of each antenna unit 42. Inan exemplary embodiment, the conducting elements for each antenna unit42 comprise: four straight, transversely extending directors 44 whichare 0.408 wavelength long and are spaced sequentially 0.25 wavelengthapart, one generally C-shaped reflector 46 which is 0.5 wavelength longspaced 0.25 wavelength from the last director 44 and one transverselyfed-element 48 which is 0.45 wavelength long and has a feed spacing of0.25 wavelength. These conducting elements are constructed by depositinga 1 millimeter coating of silver on the base cloth of the antenna unit42. A layer of silicone is deposited over the silver for oxidationprotection.

The pattern of an antenna unit 42 is: ##EQU1## where E₁ and E₂ are thehorizontal and vertical element factors, respectively

D_(r) =2πD/λ=The electrical spacing between elements.

n=#of elements

s=element spacing

δ=π/n

D=λ/2=Spacing between cells 40.

φ=deviation from array centerline.

The field pattern is identical for both polarizations.

Referring again to FIG. 3, the complete array loop 12 consists of thetwo rows of antenna cells 40 referred above, with each row comprising176 such cells twenty-two segments of eight cells each. Antenna cells 40are spaced one half wavelength apart in the horizontal dimension and sixtenths wavelength apart in the vertical dimension. The structure of cell40 is maintained in the deployed configuration by tension in the basematerial of the cell and by tension in the main booms 14.

FIG. 6 shows the RF circuit for controlling the antenna array. Thissingle RF circuit handles both transmission and reception. RF power isprovided by an exciter 68. RF power from exciter 68 is fed throughcirculator 66 to array combiner/divider 64 where the RF power is dividedfor distribution to the twenty-two antenna segments 30. Arraycombiner/divider 64 is connected to each of twenty-two segmentcombiner/dividers 62 (one of which is shown in FIG. 6). Exciter 68,circulator 66, array combiner/divider 64 and segment combiner dividers62 are mounted on or in support column 10.

Considering a typical antenna section 30, segment combiner/divider 62feeds the eight transmit/receive (T/R) modules 32 mentioned above andshown in FIG. 4. Suitable twin-lead cable (not shown) within each of themain support booms 12 transmits the RF power from each segmentcombiner/divider 62 to the corresponding T/R modules 32. As discussedabove, there are eight T/R modules 32 in each segment 30 and each module32 feeds a vertical row or pair of two antenna cells 40. As was alsodiscussed above in connection with FIG. 4, four T/R modules 32 aremounted on each main boom 14 and are connected to the antenna cells 40through flexible twin lead cable 50. In particular, each T/R module 32feeds eight antenna units 42 which form the two antenna cells 40arranged in a vertical row or pair. The wire separation of the twin leadcable 50 is adjusted for proper impedance match and equal powerdivision. As shown in FIG. 6, each T/R module 32 separately feeds thevertical polarization elements and the horizontal polarization elements.

The internal circuitry of a T/R module 32 is shown in detail in FIG. 7.RF power is received from the segment array divider/combiner 62 and istransmitted into a N-Bit phase shifter 84 via an "on/off switch" 78which provides for beam stepping. Shifter 84 is preferably a shiftregister which controls the sequence in which elements of the arrayfire, so as to steer the array's beam in accordance with well knownphased array principles. Phase shifter 84 provides phase compensationand optional vernier beam scan (±1°). Optional DPCA (Displaced PhaseCenter Antenna) operation can be controlled by having the module phaseshifters 84 produce sum and difference patterns.

Properly phase-shifted RF power is transmitted through a passivecirculator 86 into a pre-amplifier 88 and an amplifier 90. The amplifiedRF power is then transmitted through a passive circulator 96 into twoBALUNs 100. A polarization switch 98 is provided between the secondcirculator 96 and the two BALUNs 100. Each BALUN 100 provides RF powerto four antenna elements 42 (as shown in FIG. 6).

Passive circulators 86 and 96 and switches 78 and 98 provide separatetransmit and receive paths with sufficient isolation to prevent receiversaturation during the transmit cycle.

For radar reception, received RF power is transmitted from the antennaelements 42 through BALUNs 100 and polarizing switch 98 into a low noisereception-amplifier 94. Circulator 96 is properly synchronized to eitherallow transmission of power to antenna units 42 for radar broadcast or,alternately, reception of power from antenna units 42 into low noisereception-amplifier 94. Independent control of the antenna sidelobes,during the receive mode, is accomplished by a voltage controlledattenuator (not shown) in the low noise amplifier line. Receptionamplifier 94 amplifies and transmits the RF signal to N-bit phaseshifter 84 via circulator 86. N-bit phase shifter 84 shifts the phase ofthe RF signals for subsequent processing.

The RF signal is in turn transmitted from the T/R module to the segmentcombiner/divider 62 which combines the received RF signals for allantenna units 42 within one antenna segment 30. The RF signals are thentransmitted to the array combiner/divider 64 for combining with thereceived RF signals from each of the other array segments 30. Finally,the single combined RF signal is transmitted through circulator 66 toreceiver/processor 70. Circulator 66 is properly synchronized toalternately allow for RF transmission from the exciter 68 or RFreception from antenna units 42. Receiver/processor 70 provides for anynecessary processing of received RF signals and is preferably connectedto a down-link antenna (not shown) for transmitting received radarsignals to an Earth-based station.

In use, the maximum of the beam is placed near to the horizon at adepression angle of 32 degrees. The 3 dB half-power beamwidth is 32degrees. Activating 47 of the 176 total antenna cells 40 in each of thetwo rows produces the highest gain with the lowest sidelobes. Abeamwidth of 3.04 degrees is obtained with a first sidelobe at -22.4 dBand a two way sidelobe max of -44.8 dB. The total array gain at the farrange is 26.3 dB and remains constant for all azimuth stepped scanpositions. The beam can be step scanned in azimuth (one half beamwidth)for a full 360 degrees or can be slewed to any azimuth position. The 3dB antenna beam illuminates an area on the surface of the Earthapproximately 81,500 square nautical miles.

Details of the system geometry, assuming baseline antenna beamwidths of3.5° in azimuth and 30° in elevation are provided in Table I.

                  TABLE I                                                         ______________________________________                                        SBR GEOMETRY                                                                  ______________________________________                                        Satellite Altitude:     600    nmi                                            Antenna Beam Depression Angle:                                                                        32     Deg.                                           Antenna, 3 dB Vertical Beamwidth:                                                                     30     Deg.                                           Antenna, 3 dB Horizontal Beamwidth:                                                                   3.5    Deg.                                           Far Range, Grazing Angle:                                                                             5.26   Deg.                                           Far Slant Range:        1827   nmi                                            Near Slant Range:       895    nmi                                            Far Ground Range:       1607   nmi                                            Near Ground Range:      614    nmi                                            Length of Ground Spot:  994    nmi                                            Near Ground Spot Width: 55     nmi                                            Far Ground Spot Width:  112    nmi                                            ______________________________________                                    

Table II sets forth preferred system parameters for an exemplarypreferred embodiment of the Space Based Radar antenna array of theinvention.

                  TABLE II                                                        ______________________________________                                        RADAR PARAMETERS                                                              ______________________________________                                        Frequency       200     MHz                                                   Target Radar Cross-Section                                                                    100     Square Meters                                         Maximum Range to Target                                                                       1827    nmi                                                   Antenna Gain    26.3    dBi                                                   Receiver Bandwidth                                                                            1       MHz                                                   System Losses   10      dB                                                    System Noise Temperature                                                                      1000°                                                                          K.                                                    PRF             528     pps                                                   Pulse Width     190     μs                                                 Duty Cycle      0.1                                                           Pulse Compression                                                                             190                                                           Compressed Pulse                                                                              1       μs                                                 Peak Power Transmitted                                                                        7590    Kw (without pulse comp)                               Probability of Detection                                                                      0.97    (s/n = 14 dB)                                         Probability of False Alarm                                                                    10.sup.-6                                                     Dwell Time      8       sec.                                                  Peak Power/Module                                                                             850     Watts (47 modules)                                    Average Power/Module                                                                          85      Watts                                                 ______________________________________                                    

In an alternative embodiment, improved azimuth scanning is achieved byusing vernier phasers in each module 32. Lower sidelobes are achieved byusing a receive tapered combining network. Significant gain increasesare achieved with increase of aperture. In this alternative embodiment,the deployed array has a diameter of 180 feet. Nine antenna cells 40 arestacked in each vertical row (rather than two cells, as above). Five T/Rmodules are used per column of cells with one of the five T/R modulesfeeding only one cell.

This larger array includes a two-dimensional tapered distributionnetwork. A comparison of these two embodiments is shown in Table III.

                  TABLE III                                                       ______________________________________                                        HIGHER GAIN AND BASELINE ARRAY COMPARISON                                                    FIRST        HIGHER GAIN                                       ITEM           EMBODIMENT   EMBODIMENT                                        ______________________________________                                        Diameter (ft.) 150          180                                               Vertical Aperture (ft.)                                                                      7.4          24.6                                              No. of Columns of                                                                            176          230                                               Elements                                                                      No. of Rows of Elements                                                                      2V           9V                                                No. of Transmit/Receive                                                                      176          1150                                              Modules                                                                       No. of Simultaneous                                                                          47           305                                               Active Modules                                                                No. of Simultaneous                                                                          47           61                                                Active Columns                                                                Elevation Distribution                                                                       Uniform      Dolph-Tscheby,                                                                30.sub.2 dB S.L                                   Azimuth Distribution                                                                         Space-Taper  Space-Taper +                                                                 cos.sup.2, 10 dB                                                              pedestal                                          Gain (dBi)     26.3         29.9                                              Elevation, 3 dB                                                                              32           15.3                                              Beamwidth (Deg.)                                                              Azimuth, 3 dB  3.04         2.77                                              Beamwidth (Deg.)                                                              Peak Sidelobe                                                                 (one way) dBc                                                                 Azimuth        -22.4        -33.4                                             Elevation      -41.9        -29.2                                             Avg. Sidelobe                                                                 (one way), dbi                                                                Azimuth        -10.8        -16.7                                             Elevation      -24.8        -6.5                                              Avg. Power/Module,                                                                           85           8.3 with 11.5dB                                   Watts                       dynamic range                                     Weight Est. (pounds)                                                                         17,000       23,000                                            ______________________________________                                    

Although the invention has been described with respect to the exemplaryembodiments thereof, it will be understood by those skilled in the artthat variations and modifications can be effected in these embodimentswithout departing from the scope and spirit of the invention.

I claim:
 1. A spaced based radar antenna array comprising,a loopantenna; a support column having first and second ends; connectingmeans, including a plurality of support boom assemblies, for connectingsaid first end of said support column to said loop antenna; and meansfor providing movement of said boom assemblies between a first positionwherein said boom assemblies extend substantially parallel to saidsupport column such that said loop antenna is disposed in a collapsedstate in the vicinity of the support column, and a second positionwherein said boom assemblies extend outwardly substantiallyperpendicular to said support column such that said loop antenna isfully deployed in a circular configuration.
 2. The invention of claim 1,wherein said loop antenna further comprises a plurality of printedcircuit, end-fire YAGI-UDA antenna units.
 3. The invention of claim 1,wherein said means for providing movement further comprises a generallycircular runner slidably mounted to said support column such that saidrunner can move along said support column from a first position at saidfirst end of said support column to a second portion at said second endof said support column, and wherein said boom assemblies of saidconnecting means comprise:an extensor boom pivotally attached to saidrunner, said extensor boom having a center; a secondary boom connectingsaid first end of said support column to said center of said extensorboom, said secondary boom having a center; and a main supporting boomconnecting said center of said secondary boom to said loop antenna.